With direct-metal deposition, or DMD, a laser-aided, computer-controlled system applies layers of material on a substrate to fabricate an object or to provide a cladding layer. Preferably, the DMD system is equipped with feedback monitoring to control the dimensions and overall geometry of the fabricated article in accordance with a computer-aided design (CAD) description. The deposition tool path is generated by a computer-aided manufacturing (CAM) system for CNC machining, with post-processing software for deposition, instead of software for removal as in conventional CNC machining. Background for the laser-aided DMD process can be found in “Laser Material Processing,” W. M. Steen, 1998 Springer, and in U.S. patent application Ser. No. 09/107,912, filed Apr. 10, 1997, which is incorporated herein by reference.
During the process of laser-aided DMD, whether for cladding or product fabrication, residual stresses may accumulate and lead to distortion and premature failure of the finished article during its use or operation. When the accumulated residual stresses exceed the yield strength of the material, cracking often occurs during the fabrication process. Thermal expansion and sometimes phase transformation are the main contributors to residual stress. Most steels, for example, change from austenite with face-centered cubic structure (FCC) to martensite with body-centered tetragonal crystal structure (BCT) above a certain critical cooling rate. The specific volume of BCT is 4% higher than that of FCC, and therefore martensitic transformation produces considerable stress.
Although there are several techniques for post-mortem, i.e., after fabrication or after failure, measurement of stress accumulation, these techniques are not timely and do not save the product. To alleviate the problem, periodic heat treatment is often needed. On the other hand, for cyclic loading applications, compressive residual stress improves the service life. Location and relative magnitude of stress are also important for the control of distortion. Residual stress also influences hardness. Therefore, a method for tailoring the magnitude, location and direction of residual stress has a considerable potential to influence the service life and quality of components manufactured by DMD.